Method of processing transient electromagnetic measurements in geophysical analysis

ABSTRACT

A method of geophysical analysis includes transmitting an electromagnetic signal into the earth and measuring the resulting transmission of that signal through the earth to a plurality of spaced recording units. The values of the transmitted signal are locally digitized and stored at each recording unit. The stored data from each recording unit is subsequently transmitted through a telemetric link to a central processing or analyzing unit, the stored data being transmitted from one recording unit at a time and in a preselected sequence. The data may be transmitted from each recording unit in a block and stored in the central processing unit. The digitizing and storing of the measured values at each unit is accomplished through a yield point amplifier. Automatic drift correction processing may also be applied to the measured values obtained from each recording unit. A microprocessor is installed at each recording unit to process, store, and relay the data measured and to test and control the status of the receiving unit.

BACKGROUND OF THE INVENTION

The invention relates to an installation for the exploration of thesubsoil with the aid of transient electromagnetic measurements.

The invention relates specifically to the data collection of transientelectromagnetic signals, which are generated in the subsoil to beexplored by feeding through a grounded dipole.

If such signals are to supply usable information concerning thestructure of the subsoil, the always present interference sources whichdistort the signals must be avoided on the one hand, whereby, above all,alternating currents and radio signals can disturb the measuringresults. On the other hand, it is necessary to set up several remoteinterspaced measuring points. The erratic changes in the transmittingcurrent diffuse as transients in the subsoil and generate thereexpanding induction currents. The recording units contain measuringpoints which each measure different components of the electric andmagnetic fields, originating from the induction currents. The inventionpicks up from a previously known installation of this type. In thisinstallation, the measuring data are processed for the purpose ofobtaining a most favorable ratio possible of measuring data andinterference data. In the known installation, relay of the measuringdata and the recording units into a central unit is required in theoriginal form of the data. This requires that the measuring data areconveyed on separate channels and that they are digitized in the centralunit. Because only maximum eight channels can be set up, the number ofrecording units is therefor limited (U.S. Pat. Nos. 3,737,768, and4,535,293). Because the relay lines from the recording units to thecentral unit have an offset of up to 8 km depending on the requirementsof the individual case, mixing in of interference signals cannot beavoided.

It is the objective of the invention to make a more efficientinstallation, the general construction of which has been described inthe introduction, whereby especially the interference signals should besubstantially eliminated. The invention solves the objective by locallydigitizing and storing at each recording unit, the values that aremeasured at the recording unit and subsequently transmitting the storeddata from each recording unit through a telemetric link to a centralprocessing or analyzing unit, the stored data being transmitted from onerecording unit at a time and in a preselected sequence in other aspectsof the invention the stored data may be called up from each recordingunit in a block and transmitted to and stored in the central unit. Thedigitizing of the measured values at each recording unit may beprocessed by a yield point amplifier. Automatic drift correctionprocessing may also be applied to the measured values obtained at eachrecording unit. And, each recording unit may be provided with amicroprocessor to process, store, and relay the data measured at thereceiving unit and to test and control the status of the receiving unit.

According to the invention, the analog measuring values obtained by themeasuring points via sensors are immediately digitized in the allocatedrecording unit, whereby the transmission path between the sensorspicking up the measuring values and the digitizer can be kept veryshort. Transmission of the digitized signals occurs exclusively viatelemetric link (line), whereby the number of recording units can beincreased practically at random. A special advantage of the inventionlies thereby in the fact that the frequency range from 0.01 to 0.3 kHzis not subject to any basic restriction due to a number of channels. Theinstallation according to the invention can therefor carry outmeasurements without loss of quality with widely spaced out measuringpoints in a territory with strong electric interference.

The signals which are stored and digitized at the recording points aresuitably called up individually and relayed to the central station viatelemetric digital link. This embodiment of the invention permitscalling up the data blocks as needed or in preset time sequence and toplace them into the data bank of the central unit. Interpretation of thestored signals can subsequently follow, according to which aquantitative evaluation of the distribution of the electric resistancesin the subsoil is possible, whereby the depth area explored can varybetween a few meters and several kilometers.

Processing of the analog signals in the recording unit is not aprerequisite of the invention, but can occur, however, by processingwith a yield point amplifier to digitize the measured values at eachrecording unit, for example, if this should be required in theindividual case or if the user of the installation wishes this.Processing in these cases includes automatic drift correction in orderto prevent the measurements from following a distortion path. Becausethe components of the electric and magnetic fields originating from theinduction currents received at the measuring points are used, thevertical components of the magnetic field or their temporal derivativeas well as, in addition, one or several components of the electric fieldare generally measured. This permits an interpretation with the aid ofthe Maxwell equations which leads to the above described quantitativeevaluation of the electric resistances in the subsoil.

Furthermore, the invention enables the monitoring and control of therecording units, i.e. of their electronic analog components with amicroprocessor installed at each recording unit.

Below, the invention is further described using an embodiment example,which is represented in the drawing. Shown are

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 a typical field arrangement of the measuring installation,whereby however only two recording units for measuring theelectromagnetic transients have been drawn,

FIG. 2 Signal forms of the transmitting current and of the pertinenttransients received, namely E for the electric field and H for themagnetic field of the transient,

FIG. 3 a block circuit diagram of the data collection installationaccording to the invention,

FIG. 4 a block circuit diagram of the analog part of the recording unit(RU) and

FIG. 5 a block circuit diagram of the digital part of a recording unit(RU).

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows the principle of an installation for exploring the subsoilwith the aid of transient electromagnetic measurements (TEM). Theinstallation contains a transmitter (grounded dipole) and recordingunits with receivers (spool). Erratic changes in the transmissioncurrent diffuse as transients in the subsoil and there expandinginduction currents are generated. Those are received at the measuringpoints, whereby--as is known in the field of geophysical study andanalysis, is known to those of ordinary skill in the art, and isdescribed in the introduction--various components of their electric andmagnetic fields can be utilized. Examples of signal forms on thetransmitter/receiver side are shown in FIG. 2. The signals receiveddecay each time after a short period, and are therefor calledtransients. They must be interpreted with the aid of Maxwell equationsas is known in the field and known by those of ordinary skill in theart, and thus allow a quantitative evaluation of the distribution of theelectric resistances in the subsoil, which in turn provide details aboutthe make up of the subsoil.

FIG. 3 gives a block circuit diagram of the data collection systemaccording to the invention. The signals received from sensors (1,2) areprocessed, digitized and intermediately stored at the respectivemeasuring point in the recording units (3-6). The recording units arecontrolled by central station (7) via a digital telemetry system. All ofthe recording units (3-6) are linked together with this digital dataline and can be connected at random distances. The data from eachrecording unit are called up individually by the central unit andtransmitted as a complete block via the data line. Basically, anunlimited number of recording units can be connected this way.Synchronization of transmitter and recording unit occurs via an externaltrigger or via extremely accurate clocks. The analog part of a recordingunit is represented in FIG. 4 as block circuit diagram. It consists ofvarious filter and amplifier stages. Next, the signal goes through aradio frequency filter RF (9), then through a low-pass LP (12), thenthrough a receiving amplifier IA (13), in which additionally anautomatic drift correction BC (10) occurs. A notch filter NF (15)follows and finally an impedance converter IMA (16) with a second driftcontrol BC (10). Subsequently, the signal goes through a yield pointamplifier in the digital part of the recording unit. All filter andamplifier stages are microprocessor controlled. Amicroprocessor-generated test signal can be given directly to the inputfollowing digital/analog D/A (11) conversion. In this way, units (12-16) can be checked.

For drift control BC (10), the microprocessor determines correctionvalues from time windows of the input signals. The corrections are madebefore the next transient is received.

FIG. 5 shows the digital part of the recording unit. The interface tothe analog part is formed by the yield point amplifier AS (17), whichenables amplifications between 0 dB and 90 dB, in the form of anautomatic amplification regulator AGA (18) and of a 4 bit electronicexponent (19). The output of automatic amplifier AGA (18) is digitizedin the analog/digital converter ADC (20), while the 4 bit exponent istaken over directly by microprocessor MP (21). The microprocessor storesthe data in the memory and waits for the data call up by central unit DR(25), in order to transmit the data as a block into telemetry TEL (23)to central unit DR (25). Microprocessor MP (21) simultaneously controlsthe adjustment of the analog part, such as amplifier, filter and testsignal via test & control bus TCO (24).

We claim:
 1. A method for obtaining data from subsoil using ageophysical exploration installation having a transmitter in a groundeddipole, a central processing unit for processing the obtained data, anda plurality of recording units connected to the central processing unitby a telemetric link, each of the recording units having a plurality ofmeasuring points, the method comprising the steps of:transmittingelectromagnetic transients into the subsoil from the transmitter;receiving and measuring expanding induction currents using the pluralityof recording units to obtain measuring values, the expanding inductioncurrents being produced in the subsoil as a result of theelectromagnetic transients transmitted into the subsoil from thetransmitter; digitizing and storing the measuring values as stored datain each recording unit at the measuring points; and transmitting thestored data from one recording unit at a time to the central processingunit in a predetermined sequence via the telemetric link for furthergeophysical processing.
 2. The method of claim 1, wherein the step oftransmitting the data from the recording units includes transmitting thedata in blocks for subsequent storage in the central processing unit. 3.A method for obtaining data from subsoil using a geophysical explorationinstallation having a transmitter in a grounded dipole, a centralprocessing unit for processing the obtained data, and a plurality ofrecording units connected to the central processing unit by a telemetriclink, each of the recording units having a plurality of measuring pointsand a yield point amplifier, the method comprising the stepsof:transmitting electromagnetic transients into the subsoil from thetransmitter; receiving and measuring expanding induction currents usingthe plurality of recording units to obtain measuring values, theexpanding induction currents being produced in the subsoil as a resultof the electromagnetic transients transmitted into the subsoil from thetransmitter; processing the measuring values, wherein the measuringvalues are processed and digitized with the yield point amplifier;storing the digitized measuring values as stored data in each recordingunit at the measuring points; and transmitting the stored data in blocksfrom one recording unit at a time to the central processing unit in apredetermined sequence via the telemetric link for storing the datatherein and further geophysical processing.
 4. The method of claim 3,wherein the step of processing the measuring values further includesautomatic drift correction
 5. The method of claim 4, further includingthe step of installing a microprocessor at each recording unit toprocess the measuring values and to store and relay the data to thecentral processing unit, each microprocessor monitoring and controllingthe performance and settings of its respective recording unit through atest and control bus
 6. The method of claim 5 further including the stepof synchronizing the transmitter and each recording unit by one of anexternal trigger operatively linked with each of the transmitter andeach recording unit or by installation of synchronized timing devices ateach of the transmitter and each recording unit.
 7. The method of claim1 further including the step of synchronizing the transmitter and eachrecording unit by one of an external trigger operatively linked witheach of the transmitter and each recording unit or by installation ofsynchronized timing devices at each of the transmitter and eachrecording unit.
 8. A method for obtaining data from subsoil using ageophysical exploration installation having a transmitter in a groundeddipole, a central processing unit for processing the obtained data, anda plurality of recording units connected to the central processing unitby a telemetric link, each of the recording units having a plurality ofmeasuring points and a yield point amplifier, the method comprising thesteps of:transmitting electromagnetic transients into the subsoil fromthe transmitter; receiving and measuring expanding induction currentsusing the plurality of recording units to obtain measuring values, theexpanding induction currents being produced in the subsoil as a resultof the electromagnetic transients transmitted into the subsoil from thetransmitter; processing the measuring values, wherein the measuringvalues are processed and digitized with the yield point amplifier:storing the digitized measuring values as stored data in each recordingunit at the measuring points; and transmitting the stored data from onerecording unit at a time to the central processing unit in apredetermined sequence via the telemetric link for further geophysicalprocessing.
 9. The method of claim 8, wherein the step of processing themeasuring values further includes automatic drift correction.
 10. Themethod of claim 1, further including the step of processing themeasuring values by performing automatic drift correction.
 11. Themethod of claim 3, further including the step of installing amicroprocessor at each recording unit to process the measuring valuesand to store and relay the data to the central processing unit, eachmicroprocessor monitoring and controlling the performance and settingsof its respective recording unit through a test and control bus.
 12. Themethod of claim 2, further including the step of installing amicroprocessor at each recording unit to process the measuring valuesand to store and relay the data to the central processing unit, eachmicroprocessor monitoring and controlling the performance and settingsof its respective recording unit through a test and control bus.
 13. Themethod of claim 1, further including the step of installing amicroprocessor at each recording unit to process the measuring valuesand to store and relay the data to the central processing unit, eachmicroprocessor monitoring and controlling the performance and settingsof its respective recording unit through a test and control bus.
 14. Themethod of claim 1, wherein the telemetric link is a radio link.
 15. Themethod of claim 1, wherein the recording units are located on a surfaceof a geophysical area to be explored.
 16. The method of claim 1, whereinthe recording units are located on a surface of a geophysical area to beexplored and the telemetric link is a radio link.
 17. The method ofclaim 1, wherein each of the measuring points of each of the recordingunits measure different components of electric and magnetic fieldsoriginating from the expanding induction currents.
 18. The method ofclaim 3, wherein each of the measuring points of each of the recordingunits measure different components of electric and magnetic fieldsoriginating from the expanding induction currents.
 19. The method ofclaim 8, wherein each of the measuring points of each of the recordingunits measure different components of electric and magnetic fieldsoriginating from the expanding induction currents.
 20. The method ofclaim 8, wherein the recording units are located on a surface of ageophysical area to be explored and the telemetric link is a radio link.